The widget below this widget in the tree.
This widget displays child when zero drags are under way. If childWhenDragging is non-null, this widget instead displays childWhenDragging when one or more drags are underway. Otherwise, this widget always displays child.
The feedback widget is shown under the pointer when a drag is under way.
To limit the number of simultaneous drags on multitouch devices, see maxSimultaneousDrags.
This widget can only have one child. To lay out multiple children, let this
widget's child be a widget such as Row, Column, or Stack, which have a
children property, and then provide the children to that widget.
final Widget child